This application claims the priority of German Patent Application No. 197 40 926.1, filed Sep. 17, 1997, the disclosure of which is expressly incorporated by reference herein.
The invention is based on a process for the outer circumference grinding of a cylindrical journal on a workpiece. The workpiece is received in a work spindle in a rotatable manner and is rotationally drivable at a defined rotational workpiece speed and a defined circumferential speed. A grinding disk, which revolves at a defined rotational disk speed and at a defined circumferential speed and which is cylindrically trimmed on the outer circumference, is brought into a grinding engagement with the journal to be ground.
For a secure sealing function at shaft passage points through housing walls, in addition to the sealing ring provided with a ring-shaped radial sealing lip, the characteristics of the journal-side counterrotation surface must also be taken into account. As a rule, these are circumferentially ground journal surfaces. In addition to certain roughness values, the designing engineer also requires a torque free characteristic of the grinding structure for the shaft journal. A "torque free" characteristic means that the grinding structure is situated precisely in the circumferential direction and superimposed regular shaft portions are absent.
Using a rubber-elastic sealing edge, the radial sealing lip of the sealing ring rests against the surface of the shaft journal with a defined radial force and on a defined radial width. By means of the rotation of the shaft journal, the contact area of the sealing lip is deformed to a varying extent in the circumferential direction as a function of the local radial contact pressure. Smaller deformations are situated close to the edge and larger circumferential deformations are situated more in the center area of the contact strip. This results in a sensible tribological and Theological equilibrium with an oil flow which, on the one hand, ensures the lubrication of the contact zone and, on the other hand, a return mechanism which maintains the sealing function of the ring seal. This equilibrium must not be disturbed by the formation of a torque or twisting in the microstructure of the counterrotation surface. A torsional conveying effect in one or the other direction is to be avoided. In the case of a torque-induced conveying effect into the sealed interior of the housing, the seal would run dry. Exterior dirt would be conveyed into the contact zone and the seal would wear out prematurely and become leaky. Although a conveying effect directed to the outside would prevent a running-dry of the seal, it would result in a discharge of oil at the sealing point, which for various reasons must be more or less strictly rejected.
So far it has been widely assumed that the so-called plunge-cut grinding process results in torque-free structures. However, even by means of the insecure so-called thread method, it can be proven that, at least in the case of a certain combination of working parameters, also in the case of the plunge-cut process, torque structures can be formed on a workpiece surface which is finely machined in this manner. Without the targeted use of the special knowledge of the formation of twists and countermeasures derived therefrom, the plunge-cut grinding of torque-free structures is more a question of the accidental meeting of favorable process parameters. With respect to the cylindrical grinding of shaft journals, surprisingly, the formation of possible torque structures has not been considered significant or significant enough. The cause of premature seal failures is assumed to lie more with the seal and less with the journal surface and its microstructure.
Although by means of a so-called "sparking-out" after the termination of the grinding operation, as demonstrated by the applicant's investigations, a torque structure in the ground surface can be avoided, the shop term "sparking-out" relates to a continued operation, without a feeding motion, of the grinding wheel on the rotating workpiece until the emitted sparks are extinguished in the case of a dry grinding and for a correspondingly long period during a wet grinding. The longer the sparking out takes place, the lower the twist formation. However, for the torque-free sparking-out of the workpiece, it is required to maintain the operation of the sparking-out for at least 20 to 30 s. This would impair the cycle time of the grinding operation to an unacceptably high extent.
According to the applicant's experiences with a process developed by the applicant for determining torque or twist structures on finely machined cylinder surfaces, on the one hand, and with tightness and durability tests on installed seals, on the other hand, an absolute freedom from twisting is not the only device for achieving tightness and a high durability expectation on radial shaft sealing rings. Slight formations of a twist structure and/or a high number of threads with a respective low conveying cross-section also lead to tolerable results.
Twist formation during grinding takes place, on the one hand, by way of the trimming operation of the grinding wheel or by way of deviations of parallelism between the grinding wheel axis and the workpiece axis. The applicant therefore differentiates between various types of torques.
In the case of the trimming torque, a single-thread trimming spiral is first formed on the grinding disk by means of the trimming using a so-called nonwoven or using a single diamond grain. During the grinding process, corresponding to the lower rotational speed of the workpiece, this trimming spiral leaves a flatter line on the workpiece, which generally is transferred to the workpiece as a multiple-thread twist structure. In the case of a zero twist, the waviness, which is formed in the cross-sectional shape similar to that of the trimming twist, is situated precisely in the circumferential direction; that is a "twist" formation is observed which has the peculiarity that the twisting angle is precisely equal to zero. With respect to their formation, the trimming twist and the zero twist are to be assigned to the waviness and are superimposed on the grinding structure.
The grinding structure must clearly be differentiated from the waviness. The term "grinding structure" here relates to the grinding traces of the individual grains of the grinding disk on the workpiece surface. Corresponding to the circular path of the grinding disk circumference and correspondingly of the passing-by of the workpiece circumference, the abrasive grains in each case engage only temporarily with the workpiece surface. The grinding structure is formed of a plurality of surface-covering superimposed lens-shaped or fish-type notches of a length of approximately 0.5-1 mm and a width of about 1/10th of it, which are all aligned in parallel to one another. The grinding structure is therefore interrupted repeatedly and contains a high stochastic form proportion. In contrast, the waviness of the trimming twist and the zero twist is uniformly formed along the whole sealing surface and has the characteristic of an interconnection. This means that the path of a twist extends continuously along the circumference. The interconnection will exist as long as the waviness proportion of the twist is at least formed to the same extent as the roughness proportion of the grinding structure.
The cause of the offset twist is an offset angle according to DIN 8630 as a deviation from the parallelism between the axis of rotation of the grinding disk and that of the workpiece. In the microstructure of the workpiece surface, the offset twist can be recognized in that the--not interconnected--grinding structure is sloped at a small angle with respect to the circumferential direction of the workpiece. Because of the slope of the grinding structure with respect to the circumferential direction, this surface structure--irrespective of the interconnected trimming twist -, in the interaction with a sealing lip, also has an axial conveying effect which may impair the durability of the shaft sealing ring.
Irrespective of whether it is a trimming twist or an offset twist, a twist formation will impair the sealing function of the surface the more or the higher the twist angle, the lower the number of threads and the larger the surface cross-section or the depth of one or several threads or of the grinding structure. In the case of a twist structure with a low number of threads, the individual threads have the tendency to be deeper, thus larger in the cross-sectional surface than in the case of higher thread numbers. So far, a large number of sealing surfaces with a twist structure were measured and a large variety was discovered in the twist formation.
It is an object of the invention to improve the process for the cylindrical grinding of shaft journals on which this application is based in that twist structures on grinding surfaces are avoided or can at least be kept within tolerable limits without a subsequent "sparking-out" in the case of all workpieces to be machined in one cycle.
According to the invention, this object is achieved by the process for the cylindrical grinding of a cylindrical journal on a workpiece. The workpiece is received in a work spindle in a rotatable manner and is rotationally drivable at a defined rotational workpiece speed and a defined circumferential speed. A grinding disk, which revolves at a defined rotational disk speed and at a defined circumferential speed and which is cylindrically trimmed on the outer circumference, is brought into a grinding engagement with the journal to be ground. During the trimming of the grinding disk, an axial trimming advance of 0.05 to 0.15 mm per grinding disk revolution is maintained and/or while approaching the desired circumferential speed of the grinding disk and of the desired circumferential speed of the workpiece within a respective permissible range, the rotational disk speed under a load during grinding and/or the rotational workpiece speed under a load during each grinding operation is/are continuously varied and/or adjusted such that the ratio of the rotational disk speed to the rotational workpiece speed is non-integral or does not represent a simple fractional ratio.
Accordingly, the invention starts with the causes of the formation of the two different types of twists and suggests different countermeasures for avoiding twists or torques. Indirectly, the invention recommends a twist formation which is as low as possible on the grinding disk itself during trimming and a large number of threads on the workpiece-side twist structure. For this purpose, the grinding disk and the workpiece are driven at rotational speeds whose ratio is disharmonic to as high a degree as possible. This can be achieved, among other measures, by avoiding integral or simple fractional ratios of the participating rotational speeds of the grinding disk and the workpiece. Thus, ratios should also be avoided which correspond to an integral number plus the value of a simple fraction with numbers below six in the numerator and/or denominator. A change of at least one of the participating rotational speeds during a grinding operation is also conceivable for achieving this object. By this measure, a tumbling synchronization of twist threads generated on the workpiece side with the disk-side trimming spiral is to be avoided.
In German Patent document DE 37 37 641 C2, with a view to achieving optimal surface roughnesses during plane grinding, certain ratios of the circumferential speed of the grinding disk and the workpiece are to be maintained, but the technical context is completely different there than in the present case. Although the known process involves a simultaneous grinding of a cylindrical circumferential surface, on the one hand, and a wavy shoulder, on the other hand, by means of a biconically trimmed grinding disk whose axis of rotation is sloped at a large angle with respect to the workpiece axis, the known grinding disk carries out a periodical axial lift corresponding to the wave shape of the wavy shoulder. In contrast, when grinding sealing surfaces according to the invention, no axial relative movement is to take place between the grinding disk and the workpiece. The cited prior art does not indicate the problem of an insufficient sealing effect of ground cylinder surfaces and its elimination.
A careful trimming of the grinding disk with a slight advance recommended according to the invention already causes a twist structure to slightly form on the workpiece. The measure, which is to be recommended additionally or as an alternative, of providing highly fractional rotational speed ratios of rotational disk speeds to rotational workpiece speeds aims in the same direction. The more complicated the fraction of the rotational speed ratio at least at the end of the grinding operation, the more threads the twist structure will have and the weaker the construction of its individual threads.
According to the invention, the trimming twist can be reduced at least to tolerable measurements by the careful trimming of the grinding disk and/or by avoiding integral or simple-fraction ratios of the rotational speeds. However, independently of the above, a possible offset twist will still exist which is caused by a parallelism defect of the grinding disk axis and the workpiece rotation axis. An occurring offset twist can be avoided in that the cause of its formation is eliminated; that is, that the axis of rotation of the workpiece and that of the grinding disk are aligned precisely in parallel to one another.
The advantages of the invention are, that through the use of the targeted adjustment of machine parameters during plane or cylindrical grinding, a twist structure on finely machined journal surfaces can be avoided or be kept within tolerable limits, without any increase of the machining time.
After the above, more general explanations concerning the invention, the measures will be explained in the following which are possible according to the invention for avoiding or reducing twists, partly by means of a numerical example and partially with reference to the technological sequences.